US8981303B2 - Sensor device - Google Patents

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US8981303B2
US8981303B2 US13/795,978 US201313795978A US8981303B2 US 8981303 B2 US8981303 B2 US 8981303B2 US 201313795978 A US201313795978 A US 201313795978A US 8981303 B2 US8981303 B2 US 8981303B2
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terahertz wave
optical waveguide
light
sensor device
coupler
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US20130240740A1 (en
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Toshihiko Ouchi
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Canon Inc
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Canon Inc
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/08—Optical arrangements
    • G01J5/0818—Waveguides
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/10—Arrangements of light sources specially adapted for spectrometry or colorimetry
    • G01J3/108—Arrangements of light sources specially adapted for spectrometry or colorimetry for measurement in the infrared range
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/55—Specular reflectivity
    • G01N21/552—Attenuated total reflection
    • G01N21/553—Attenuated total reflection and using surface plasmons
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation
    • G01N21/3586—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation by Terahertz time domain spectroscopy [THz-TDS]

Definitions

  • the present invention relates to a sensor device using a terahertz wave containing electromagnetic wave components in a frequency domain from a millimeter wave band to a terahertz wave band (not less than 30 GHz and not more than 30 THz), and a sensing system or an imaging system using the same.
  • terahertz waves terahertz waves
  • spectroscopic technology for determining the absorption spectrum or complex permittivity inside a substance to examine physical properties such as the bonding state of molecules
  • measurement technology for examining physical properties, such as carrier concentration or mobility, and electric conductivity, and biomolecule analysis technology.
  • a method using nonlinear optical crystal is widely used.
  • Typical nonlinear optical crystals include LiNbOx (hereinafter, also referred to as LN), LiTaOx, NbTaOx, KTP, DAST, ZnTe, GaSe, GaP, and CdTe. Second-order nonlinear phenomena are used for generation of a terahertz wave.
  • LN LiNbOx
  • NbTaOx LiTaOx
  • KTP LiTaOx
  • DAST difference-frequency generation
  • ZnTe GaSe
  • GaP GaP
  • CdTe Second-order nonlinear phenomena are used for generation of a terahertz wave.
  • DFG difference-frequency generation
  • the energy state is excited by the incidence of the laser beams and an energy wave is radiated when returning to the original energy state.
  • an energy wave corresponding to the polarized frequency is radiated, while when it is polarized with a frequency of a terahertz wave, the terahertz wave is radiated from the nonlinear optical crystal.
  • a system for generating a monochromatic terahertz wave by an optical parametric process and a system for generating a terahertz wave pulse by optical rectification with radiation of a femtosecond pulsed laser beam.
  • electro-optic Cerenkov radiation As a process of generating a terahertz wave from such nonlinear optical crystal, electro-optic Cerenkov radiation has recently drawn attention. This is a phenomenon in which, as illustrated in FIG. 8 , a terahertz wave 101 is radiated in a conical shape like a shock wave when a group velocity of propagation of a laser beam 100 as an excitation source is faster than a propagation phase velocity of the generated terahertz wave.
  • v g and n g denote the group velocity and group refractive index, respectively
  • v THz and n THz denote the phase velocity and refractive index of the terahertz wave, respectively.
  • a monochromatic terahertz wave is generated by a DFG system using a slab waveguide having a thickness sufficiently smaller than the wavelength of the generated terahertz wave to eliminate the necessity of wavefront tilt (Japanese Patent Application Laid-Open No. 2010-204488 (Patent Document 1)).
  • the examples described in the aforementioned conventional art documents are related to a proposal of performing phase matching in the radiation direction between terahertz waves generated from different wave sources because the terahertz waves are generated by traveling-wave excitation to reinforce the terahertz waves with each other in order to improve extraction efficiency.
  • a terahertz wave generated from a slab waveguide propagates in an adjacent coupler (Si prism in the case of Patent Document 1) and is extracted from the coupler into a space.
  • the features of this radiation system include the fact that a high-intensity terahertz wave can be generated with relatively high efficiency among those using nonlinear optical crystal, and the fact that the terahertz wave band can be widened by selecting absorption in a terahertz region due to a phonon resonance unique to the crystal on a high frequency side.
  • these techniques can widen the generation band, and in the case of generating a terahertz wave pulse with optical rectification, the pulse width can be reduced. Therefore, it is expected that the device performance can be improved when the device is used in a terahertz time-domain spectroscopic apparatus, for example.
  • Cerenkov radiation of a terahertz wave generated in nonlinear optical crystal (which is the term used in these documents, and in this specification, the term “electro-optic crystal” as an approximately equivalent term is used) and propagating in the coupler is all extracted into a space. Then, light is focused on a sample desired to be sensed as necessary by means of a parabolic mirror or a lens to analyze a microscopic region of the sample.
  • the wavelength of a terahertz wave used is typically about a few hundred ⁇ m, light can only be condensed up to a beam diameter corresponding to the wavelength due to the diffraction limitation.
  • the spatial resolution is generally in millimeters though it depends on the optical system. This makes it difficult to sense a microscopic sample or to deal with imaging of a component distribution at a resolution equal to or less than the wavelength. To respond to a request for observation at an improved spatial resolution, it is necessary to use known near field technology in an optical region.
  • a sensor device has an optical waveguide containing electro-optic crystal for propagating light, a coupler provided adjacent to the optical waveguide to propagate a terahertz wave generated from the electro-optic crystal as a result of the propagation of light in the optical waveguide, and a detector for detecting the terahertz wave propagating through the coupler or the light propagating through the optical waveguide, wherein the terahertz wave is totally reflected in a section of the coupler opposite to a section where the coupler is adjacent to the optical waveguide while passing through and propagating in the optical waveguide, and in the total reflection section, the terahertz wave interacts with a subject placed close to the total reflection section.
  • the sensor device is so designed that the terahertz wave interacts with the subject in the total reflection section of the coupler, so that a microscopic region equal to or less than the wavelength can be analyzed by using the terahertz wave.
  • a minute amount of sample can be analyzed or the subject can be scanned by attaching the sensor device at one end of a probe, resulting in imaging at high spatial resolution.
  • FIG. 1 is a structural diagram of a sensor device according to Embodiment 1 of the present invention.
  • FIG. 2 is a block diagram of a terahertz time-domain spectroscopic apparatus according to Embodiment 1 of the present invention.
  • FIG. 3 is a chart illustrating an example of temporal waveform acquired in Embodiment 1.
  • FIGS. 4A and 4B are structural diagrams of a sensor device according to Embodiment 2 of the present invention.
  • FIG. 5 is a diagram illustrating a general structure of Embodiment 2.
  • FIG. 6 is a structural diagram of a sensor device according to Embodiment 3 of the present invention.
  • FIG. 7 is a structural diagram of a sensor device according to Embodiment 4 of the present invention.
  • FIG. 8 is a conceptual diagram of electro-optic Cerenkov radiation.
  • a terahertz wave is totally reflected by an interface between a coupler for extracting the terahertz wave generated by light propagating through the optical waveguide and the outer side of the coupler to propagate in the coupler. Then, the terahertz wave propagating through the coupler or the light propagating through the optical waveguide is detected.
  • a measurement sample is placed on an interface with the coupler where the terahertz wave is totally reflected so that a detector will detect that the propagation state of the terahertz wave in the coupler varies.
  • the detector detects changes in the propagated light interacting with the terahertz wave the propagation state of which varies to acquire sample information.
  • the variations in the propagation state of the THz wave occur in such a manner that the THz wave is totally reflected in a section of the coupler opposite to a section of the coupler adjacent to the optical waveguide while passing through and propagating in the optical waveguide, and in the total reflection section, the THz wave interacts with a subject placed close to the total reflection section.
  • the terms “sample” and “subject” are used as almost synonymous terms to express an object to be sensed.
  • an evanescent field of the terahertz wave is formed close to the totally reflecting interface, and the electric field does not generally penetrate the outside of the coupler beyond a distance about one tenth of the wavelength (wavelength of the terahertz wave in a free space).
  • this penetration depth exactly follows a known theoretical formula determined by the refractive indexes of two mediums that form the interface, and the incident angle and wavelength of a totally reflected electromagnetic wave with respect to the interface. Therefore, it is necessary to place the sample to almost contact or contact with the totally reflecting interface, i.e., to put the sample close to the totally reflecting interface up to a distance of the penetration of the electric field (penetration depth) or less.
  • This “almost contact” may be direct or indirect through a film of glass or resin. Further, an aperture corresponding to the wavelength or less (see a window structure in FIG. 4 to be described later) is provided as necessary so that imaging at a spatial resolution of the wavelength or less can be performed.
  • the coupler is provided adjacent to the optical waveguide so that the terahertz wave generated in the optical waveguide can be extracted into the coupler. The meaning of this term “adjacent” is defined from the standpoint of the degree of intensity attenuation of propagation light at least on the interface between the coupler and the optical waveguide as will be described later.
  • a terahertz sensor device made of LN crystal (one kind of electro-optic crystal already illustrated) according to Embodiment 1 of the present invention will be described with reference to FIG. 1 .
  • a layer 1 made of MgO doped LN crystal is sandwiched between two low refractive layers 2 and 8 as oxide film layers or resin layers to form a slab optical waveguide.
  • This is an optical waveguide containing electro-optic crystal for propagating light.
  • the thicknesses of the respective layers are typically as follows: The LN crystal layer is 3.8 ⁇ m, and the low refractive layer is 0.5 ⁇ m to 2.5 ⁇ m.
  • the orientation of the LN crystal is determined by transferring a Y-cut LN crystal substrate to a Si substrate (including processes such as adhesion and grinding so that the propagation direction of incident light 4 will be X axis and the upward direction in FIG. 1 will be Y axis. This is because, when the direction of the electric field of the incident light 4 illustrated in FIG. 1 is z-axis normal to the plane of paper, the electric field acts on z-axis where the nonlinear coefficient of the LN crystal is large to maximize the generation efficiency of the terahertz wave.
  • the orientation of the LN crystal may be any other crystal orientation.
  • Such a structure can efficiently generate the terahertz wave by electro-optic Cerenkov radiation as the second-order nonlinear phenomena.
  • the generated terahertz wave is intensified in a direction where the Cherenkov angle determined by a difference in refractive index between light and the terahertz wave in the LN crystal is about 65 degrees.
  • the coupler made of a material having a refractive index with which the terahertz wave can propagate properly is arranged adjacent to the optical waveguide, the direction will be substantially determined by a ratio between the group velocity of the terahertz wave in the material and the group velocity of light in the optical waveguide.
  • a coupler 3 for propagation of the terahertz wave is arranged adjacently on the upper surface of the low refractive layer 2 .
  • the coupler 3 is provided adjacent to the optical waveguide so that light will propagate in the optical waveguide to propagate the terahertz wave generated from the electro-optic crystal.
  • Si is suitably used for the coupler in terms of the magnitude of the refractive index and low loss of the terahertz wave, but the material is not limited thereto.
  • an ultrashort pulse laser beam of about 100 fs is typically entered into the optical waveguide to generate a terahertz wave pulse from the optical waveguide containing the LN crystal by the optical rectification effects.
  • the pulse width be narrowed because the peak intensity increases as the pulse width becomes narrower to enhance the nonlinear phenomenon.
  • a pulse width of 30 fs or less is suitably used.
  • the above-mentioned Si substrate transferred from the LN crystal layer can be used intact as the coupler.
  • the terahertz wave propagates in a zig-zag manner as indicated by the path 5 in FIG. 1 to be totally reflected on an interface between the bottom part of the low refractive layer 8 and space while being partially penetrated into the interface between the coupler 3 and the space, and into the optical waveguide.
  • the Si coupler for the LN crystal if the propagation distance of light in the optical waveguide in FIG.
  • the difference in propagation time is as follows: Since the ratio between the refractive index of the LN crystal to the light and the refractive index of Si to the THz wave is 2.2:3.4, the ratio between the propagation velocities of v light and v THzwave becomes 3.4:2.2.
  • the sensor device of the embodiment is so designed that the terahertz wave generated from the electro-optic crystal and the terahertz wave reflected in a total reflection section of the coupler will not interfere with each other.
  • the thickness is comparable to that commonly available as Si wafer, it could be said that the thickness has a strength enough for the thin-film LN crystal to support the optical waveguide.
  • the terahertz wave propagating through this coupler is converted to an electric signal by a terahertz wave detector 7 integrated at an end having a face where the propagated terahertz wave is not totally reflected.
  • the terahertz wave detector 7 for example, a known photoconductive device made of low-temperature grown GaAs can be used.
  • a pulse waveform can be acquired in a terahertz time-domain spectroscopic apparatus (THz-TDS) as illustrated in FIG. 2 .
  • the electro-optic crystal it is also possible to use the electro-optic crystal to detect the terahertz wave. As illustrated in FIG. 1 , it is desired that an end face not to meet a total reflection condition (e.g., to be normal to the Cherenkov angle of 49 degrees) be formed in an end portion of the coupler to paste the terahertz wave detector on this end face.
  • a total reflection condition e.g., to be normal to the Cherenkov angle of 49 degrees
  • the detector is bonded to a surface that meets the total reflection condition (not illustrated) without creating such a cut face to detect an evanescent wave directly.
  • a photodetector detects light propagating in the optical waveguide. In the case of a photodetector, a pin photo diode or the like may be pasted directly on the exit end face of the optical waveguide.
  • plural terahertz waves that reach the detector 7 late are detected for a single incident laser pulse. Since this depends on a difference in propagation time determined by the materials and thicknesses of respective elements described above, plural terahertz waves can be detected at time intervals determined for each individual sensor device if no sample is placed.
  • a signal is detected twice at each time interval ⁇ t after a terahertz wave that reaches without being reflected even once.
  • second and third pulses reach the detector 7 while being deformed because of a difference in reflection coefficient due to a difference in complex permittivity of the sample upon reflection on a certain face of the sample. Since the first pulse reaches directly from the optical waveguide, the first pulse can be used as reference for observation of pulse modifications of the second and third pulses.
  • a pulse signal of terahertz wave that is not subjected to total reflection in the coupler and a pulse signal of terahertz wave that is subjected to total reflection in the coupler are detected by the detector.
  • the signal that is not subjected to total reflection is compared with the signal that is subjected to total reflection.
  • a third step from the comparison result in the comparison step, information on a change caused by the presence or absence of the placement of the subject in the total reflection section of the coupler is obtained to acquire the properties of the subject.
  • the thickness of the Si coupler 3 is set to 0.58 mm as mentioned above and the length of the optical waveguide can be set, for example, to about 2.5 mm. Attempts of sample detection using the sensor device of the present invention will be described in an example later.
  • the thickness necessary for an electro-optic crystal section of the optical waveguide is equal to or less than half the equivalent wavelength in the device to the maximum frequency of a terahertz wave to be extracted (i.e., such a thickness that does not cause cancellation of reversed phases after phase shifting corresponding to the thickness of a core portion 1 is reversed on the equiphase surface of the generated terahertz wave).
  • each of the upper and lower low refractive layers 2 and 8 be thick enough to function as a clad layer during propagation of a laser beam through the optical waveguide and thin enough to be able to ignore the influence of multiple reflection or loss during propagation of the terahertz wave in the coupler 3 .
  • the light intensity on the interface with the coupler 3 in the optical waveguide having the low refractive layer 2 as a clad be equal to or more than a thickness equal to or less than 1/e 2 (where e is a base of natural logarithm) of the light intensity in a region where the crystal 1 is a core region.
  • the coupler is arranged adjacent to the optical waveguide so that the light intensity on the interface between the coupler and the optical waveguide will be as mentioned above.
  • the thickness of the upper clad layer be a thickness of about one tenth of an equivalent wavelength ⁇ eq (THz) of the terahertz wave in the low refractive layer 2 at the maximum frequency for external radiation.
  • THz equivalent wavelength
  • any structure in the size one tenth of the wavelength is generically considered to be able to ignore the influences of reflection, scattering, and refraction on an electromagnetic wave of the wavelength.
  • the thicknesses of the LN crystal layer and the low refractive layers mentioned above are derived from this design concept.
  • the wavelength of the terahertz wave in the free space will be about 39.5 ⁇ m.
  • the optical waveguide contains the electro-optic crystal as a core portion of light and the low refractive layers as a clad portion.
  • the thickness d of at least one layer fulfills a ⁇ d ⁇ eq /10, where a thickness that is 1/e 2 of the intensity of light propagating in the electro-optic crystal is denoted by a, and the equivalent wavelength in at least one layer at the maximum frequency of the terahertz wave propagating in the coupler is denoted by ⁇ eq .
  • the electro-optic crystal to form the optical waveguide used to generate a terahertz wave is not limited to the LN crystal.
  • LiTaOx, NbTaOx, KTP, DAST, ZnTe, GaSe, GaP, and CdTe can also be used as other kinds of electro-optic crystals.
  • Si is suitably used as the material used for the coupler, but any other material may be selected to make a combination of a coupler having a refractive index, with which the terahertz wave can propagate in the coupler, and the electro-optic crystal.
  • Ge can also be used for the coupler.
  • the number of reflections of the terahertz wave, the thickness of the coupler, the length of the optical waveguide are not limited to those in the embodiment.
  • such a structure to increase or decrease the number of pulses can be designed by increasing or decreasing the number of reflections, i.e., by increasing or decreasing the ratio between the length of the optical waveguide and the thickness of the coupler, and such a structure to increase or decrease the pulse time intervals can be designed by increasing or decreasing the thickness of the coupler.
  • the principles are as already described. Further, the specifications for the pulse width of a light pulse to avoid interference among plural pulses, the repetition time, and the like are also determined by the structural design.
  • FIG. 2 illustrates Example 1 in which a terahertz time-domain spectroscopic system (THz-TDS) is configured by using the sensor device of Embodiment 1.
  • a femtosecond laser 20 containing optical fiber to emit an ultrashort pulse in femtoseconds is used as an excitation light source to split light into pump light 22 and probe light 23 through an optical splitter 21 .
  • a femtosecond laser of which the center wavelength is 1.55 ⁇ m, the pulse width is 20 fs, and the pulse repetition frequency is 50 MHz is used, but the wavelength may be in 1.06 ⁇ m band or the like, and the pulse width and the pulse repetition frequency are not limited to these values.
  • the pump light 22 is coupled to a waveguide containing the above-mentioned electro-optic crystal 1 of the sensor device 24 according to the present invention.
  • a lens 25 is used, but a SELFOC (registered trademark) lens may be integrated by bonding the SELFOC lens to the incident end face of the sensor device 24 .
  • the application of non-reflecting coating to the end portion leads to reduction in Fresnel loss and reduction in unwanted interference noise.
  • output of the femtosecond laser 20 may propagate through optical fiber (not illustrated) so that bonding is made as butt coupling to butt the fiber and the waveguide of the sensor device 24 .
  • an adhesive can be selected appropriately to reduce adverse effects of reflection.
  • a fiber type can be used.
  • a fiber portion that cannot maintain polarization is contained in the above fiber (not illustrated) or the femtosecond laser 20 .
  • the excitation light source is not limited to the fiber laser. If the laser is not the fiber laser, measures taken to stabilize polarization and the like will be reduced.
  • the generated terahertz wave propagates in the coupler of the sensor device 24 to enter a detector 29 .
  • the photodetector is a photoconductive device with a dipole antenna formed in low-temperature grown GaAs
  • the wavelength of excitation light from the light source 20 is 1.55 ⁇ m
  • unillustrated SHG crystal will be used to create a harmonic as probe light 23 of the detector 29 .
  • the laser output is sufficient, a mixing phenomenon of two-photon absorption and middle level transition can lead to direct excitation with light of 1.55 ⁇ m without using the SHG crystal, which is practical.
  • a fundamental can be used for probe light without creating the harmonic in the detector 29 of the photoconductive device made of an InGaAs single layer or MQW.
  • a GaAs system can also be used in 1 ⁇ m band without the SHG crystal.
  • an optical chopper is arranged on the probe light side to modulate the light to enable synchronous detection using a signal acquiring section 26 for acquiring a detected signal through an amplifier (not illustrated) from the detector 29 .
  • a PC or the like is used to acquire the waveform of a terahertz signal while controlling an optical delay device 27 as a delay section to move.
  • the delay section may be of any type as long as the delay section can adjust a delay time between the generation of a terahertz wave in the sensor device 24 and the detection of the terahertz wave in the detector 29 as detection means.
  • the structure as mentioned above can detect a terahertz wave generated and propagated in the sensor device, and acquire information on a sample by analyzing terahertz light interacting with the sample placed on the sensor device 24 .
  • the system for sensing or imaging in the example includes the sensor device according to the present invention, a delay section for adjusting the delay time between the generation of a terahertz wave in the optical waveguide and the detection of the terahertz wave in the detector, a light source for generating light propagating in the optical waveguide, and a processing section for acquiring a terahertz wave signal interacting with a subject from output of the detector to perform processing.
  • DNA prepared in 0.5 ⁇ g/ ⁇ l as liquid samples is used to attempt a structural determination of double strand (ds) and single strand (ss).
  • the samples used are 5.4 kb circular double-stranded plasmid DNA and a single strand generated by heat-denaturing the DNA at 95 degrees Celsius for three minutes.
  • characteristic optical spectra are not observed, the samples can be sensed because the time intervals of the second and third pulses are different in FIG. 3 due to a difference in permittivity between the samples.
  • the above describes the example of the determination of DNA samples by including the entire system configuration, but the femtosecond laser and the system used for the THz-TDS apparatus are not limited to those described here as long as signals of a time-domain spectroscopic system can be obtained.
  • a tablet, powder, a liquid solution, a tissue section, and the like which can be placed to almost contact with a total reflection region of the coupler (the meaning of “almost contact” is as mentioned above), can be measured as a sample.
  • the determination method may be other than the method of making a determination from the time shifts of pulses in the example, such as to make a comparison in terms of amplitude variations in pulses or to perform Fourier transform on the pulses to perform spectral analysis. At this time, if the sample has a characteristic fingerprint spectrum, the component of the sample can be identified by a known method.
  • the size of a sample is assumed to be the size placed in a region of about 1 mm.
  • a sample of 100 ⁇ m or less can be sensed even if the sample is placed on the reflecting surface of the coupler. This is because, when the coupler is made of Si, the refractive index is 3.4 and this can form a pulse having a center frequency of 1 THz in the shape of a 100- ⁇ m spot or smaller on the reflecting surface due to the wavelength reduction effects.
  • a liquid reservoir structure (not illustrated) of about 100 ⁇ m ⁇ may be made of resin or the like to prevent the sample from spreading peripherally or the sample may be supplied after an absorber into which the liquid sinks (such as a sponge-like structural zone having multiple minute pores, not illustrated) is arranged. In this case, a microscopic region can be sensed compared to sensing in normal space.
  • Embodiment 2 is to assemble a probe structure as illustrated in FIGS. 4A , 4 B, and 5 .
  • the structure of a probe tip will be described with reference to FIGS. 4A and 4B .
  • an elongated structure is illustrated as a probe, but this shape can vary, such as the shape of a short column.
  • FIG. 4A is a sectional view in which a window structure (aperture) 31 is provided at the tip of a probe outer frame 32 and pressed against the surface of a subject (sample) 30 .
  • the window structure is typically a round aperture of 100 ⁇ m ⁇ in diameter, but the shape and size of the aperture are not limited thereto.
  • the round aperture is generally suited to a case where the terahertz wave is non-polarized light, it can, of course, be applied to a case where the terahertz wave is polarized unidirectionally.
  • a rectangular shape such as a square is suited to a case where the terahertz wave is polarized in a certain direction. Therefore, the aperture may be a rectangular aperture.
  • the size is decreased as requirements for resolution performance increase. Thus, it is only necessary to set the size on a case-by-case basis.
  • the window structure may be a glass or resin plate the thickness of which is set properly so that an evanescent wave will penetrate into the outside. This makes the inside of the probe hermetically closed to prevent contaminations from the outside.
  • an optical waveguide 39 made up of electro-optic crystal and low refractive layers like in Embodiment 1, and a sensor device 33 made up of a coupler 38 for propagation of a terahertz wave are arranged to be pressed against the outer frame 32 by two pieces of optical fiber 34 each having a core 35 and a clad 36 . It would be better to bond the entire structure after being assembled by an adhesive for mechanical stability.
  • the structure is designed to reflect a terahertz wave once on a sensing surface (total reflection surface) up to a terahertz wave detector 37 .
  • the design principles are as described in Embodiment 1.
  • the structure is designed according to the refractive indexes of materials used for the optical waveguide 39 and the coupler 38 .
  • this sensor device 33 may be of the same type as that in Embodiment 1, the optical waveguide here is of a ridge type, rather than a slab type, as illustrated in a perspective view of FIG. 4B to enhance the spatial resolution in order to improve output of the terahertz wave.
  • the transverse structure of the optical waveguide can be so constructed that a surrounding region 41 is made by Ti diffusion to have a high refractive index and a waveguide core 40 is formed into a ridge shape by a method of providing a difference in refractive index or etching. A method of filling the surrounding region 41 with resin or the like may also be used. Further, a low refractive layer 42 is inserted between the coupler 38 and the core 40 .
  • the width of the core is set, but not limited, to 4 ⁇ m to enable a single mode for propagating light.
  • the generated terahertz wave is radiated from the optical waveguide in the traverse direction.
  • the thickness of the coupler 38 is 150 ⁇ m and the length of the bottom face to contact with the optical waveguide is about 450 ⁇ m.
  • the end face is cut to have an inner angle of 41 degrees (cut to exit vertically with respect to a Cherenkov angle of 49 degrees).
  • the above window structure 31 is so set that the center will come to a point of one reflection.
  • a face of the coupler 38 on the incident side of a laser beam is, but not limited to, normal to the direction of the optical waveguide as illustrated in FIGS.
  • the thickness of the coupler 38 is made thin (150 ⁇ m) so that a radiation region of the terahertz wave from the ridge-shaped core 40 toward the window structure 31 should not be too large. This increases the spatial resolution. It is also desired that appropriate cut structures or non-reflecting coating be applied to the other end faces of the coupler 38 so that the reflection on the end faces will not affect the detected signals.
  • an end face of the optical waveguide 39 on which a laser beam is incident is cut to 45 degrees so that light from the core 35 of the optical fiber can be coupled as pump light.
  • the optical waveguide 39 has a diagonal cut face for reflecting light to change the propagation direction of the light propagated from the outside in order to couple the light to the optical waveguide. It is also desired that even a terminal portion of the optical waveguide 39 be cut diagonally as illustrated in FIGS. 4A and 4B to avoid multiple reflections inside the optical waveguide 39 , or be subjected to non-reflecting coating or surface roughening.
  • Light from the other piece of fiber 34 is radiated to the detector 37 by means of a micromirror 45 to act as probe light.
  • a laser beam from a femtosecond laser 53 is split by a beam splitter 56 into pump light 54 and probe light 55 , and coupled to the two pieces of optical fiber 34 inside a terahertz wave probe 52 .
  • the optical fiber 34 is connected to a sensor device using a terahertz wave at a tip 51 of the terahertz wave probe 52 .
  • An optical delay system 57 is arranged to operate as THz-TDS like in Embodiment 1.
  • the tip 51 of the terahertz wave probe 52 is pressed on a subject, e.g., an antebrachial region 50 of a human body to conduct an examination.
  • the contact-type terahertz imaging probe of the embodiment includes the sensor device of the present invention with the aperture at one end to face a subject. Then, a waveguide for guiding a wave to this optical waveguide of the sensor device and electric signal wiring electrically connected to the detector of the sensor device are incorporated.
  • a spatial optical system is illustrated as the optical system for guiding light to the optical fiber 34 , but the femtosecond laser 53 can be a fiber laser of an all-fiber type that connects the output to the terahertz wave probe 52 all the way through the fiber.
  • an optical delay system inside the laser can be used as a time lag between pump light and probe light.
  • ahertz wave probe 52 When the terahertz wave probe 52 is pressed on the antebrachial region 50 to conduct an examination, imaging is performed to assist in the diagnosis of cutaneous inflammation, disease, cancer, or the like.
  • data on signal variations associated with a disease previously acquired is stored as a database to compare.
  • the comparison results are processed to enable high-speed imaging such as a determination on physical properties.
  • a structure is also effective in the following case: When a transdermal drug is administered as a drug delivery system, the infiltration condition is observed in a non-destructive manner.
  • this structure can be applied to a case where the lining of an internal organ is observed by introducing an endoscope in the body and a case where tissues being treated during a surgery and the vicinity thereof are observed.
  • a window structure like in the embodiment can be used to perform terahertz imaging at a spatial resolution of the wavelength of a terahertz wave or less.
  • Embodiment 3 is a modification to Embodiment 1.
  • an electro-optic crystal (e.g., MgO doped LN crystal) 60 and an optical waveguide made up of two low refractive layers 61 and 63 are arranged in the same way as in Embodiment 1.
  • a coupler 64 for propagation of a terahertz wave has an inclination, and a crystal substrate (e.g., LN crystal) 62 of the same kind as the electro-optic crystal 60 of the optical waveguide to support the optical waveguide is provided.
  • the total reflection section of the coupler 64 includes an inclined face that is not parallel to the optical waveguide.
  • the sensing surface of the coupler can have plural inclinations to be changed along the way.
  • the coupler may have both a face parallel to the optical waveguide and an inclined face.
  • the sensing surface may be so formed that a parallel plane is changed to an inclined plane halfway.
  • the structure of the coupler 64 and the presence or the absence of the substrate 62 can be combined independently. For example, there is a case where the coupler is flat with a substrate, or a case where the coupler is inclined without any substrate.
  • the terahertz wave is S-wave like in Embodiment 1 (where the Z axis in FIG. 1 is the electric field direction)
  • a reflectivity of about 30 percent can be achieved.
  • Such a structure can be applied to a case where the coupler is too thin to keep the strength or a case where the waveguides are created on the same crystal substrate to improve reliability.
  • the inclination of the sensing surface of the coupler can be set to any angle within the scope of the present invention as long as terahertz propagation can occur properly in the coupler.
  • Embodiment 4 according to the present invention is to arrange a conductive layer of metal or semiconductor having high electric conductivity on the top of the sensing surface in order to use the operation of a known plasmon sensor together.
  • the plasmon sensor is a sensor using such a phenomenon that, when the conditions for adequate incident angle and refractive index to the total reflection surface are met, a surface plasmon resonance is induced to make a reflected wave decrease sharply.
  • FIG. 7 illustrates the former example.
  • FIG. 7 illustrates only the main part of the total reflection section. Since the other components such as the detector can be selected properly from the aforementioned embodiments, the illustration thereof is omitted.
  • the sensing surface can be inclined beforehand as in Embodiment 3 to cause a plasmon resonance for a sample 74 to be examined.
  • a coupler 73 is arranged adjacent to an optical waveguide containing an electro-optic crystal 70 and low refractive layers 71 and 72 .
  • MgO doped LN crystal can be used as the electro-optic crystal 70 and Si can be used for the coupler 73 .
  • a conductive layer 75 is formed on the surface of the Si coupler 73 by ion implantation or the like.
  • a metallic film may be formed by vapor deposition or the like to form this conductive layer.
  • the sample 74 is placed in a region where a pulse of terahertz wave generated in the optical waveguide reaches at the Cherenkov angle, and is totally reflected so that the sample can be sensed. It should be noted that the case where the coupler is not inclined and other modifications such as the Otto configuration fall within the range of forms that can be carried out as the present invention.
  • the attenuation extinction ratio of a terahertz wave signal to a sample having a specific refractive index can be increased to improve sensitivity.
  • the description is mainly made by taking, as an example, the case where a femtosecond laser beam is used as excitation light to generate a pulse of terahertz wave by optical rectification.
  • a difference-frequency generation system for entering laser beams having two different oscillation frequencies ⁇ 1 and ⁇ 2 to output a monochromatic terahertz wave corresponding to the difference frequency may be employed.
  • interference with the above-mentioned plural propagation paths occurs in places and beats of the terahertz waves are observed.
  • the terahertz waves interact with a subject in a manner as mentioned above to change the propagation state, changes in terahertz waves can be detected to acquire information on the subject.
  • the laser light source Nd:YAG laser-excited KTP-OPO (Optical-Parametric-Oscillator) light source (which outputs two wavelengths of light) or two wavelength-variable laser diodes can be used.

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